redirect_update_8103() fails when loading YAML config

Problem/Motivation

When updating from an early version of 8.x-1.x, update hook redirect_update_8103() fails when schema updates are ran.

Proposed resolution

The contents of the YAML file need to be assigned to a variable and passed to Yaml::parse()

Documentation for the Symfony YAML parser

/**
 * Parses YAML into a PHP value.
 *
 *  Usage:
 *  <code>
 *   $array = Yaml::parse(file_get_contents('config.yml'));
 *   print_r($array);
 *  <\/code>
 *
 * @param string $input A string containing YAML
 * @param int    $flags A bit field of PARSE_* constants to customize the YAML parser behavior
 *
 * @return mixed The YAML converted to a PHP value
 *
 * @throws ParseException If the YAML is not valid
 */
public static function parse($input, $flags = 0)
{
}

Remaining tasks

None.

User interface changes

None.

API changes

None.

Data model changes

None.

Release notes snippet

Fixes a critical error for redirect_update_8103() that wasn't properly loading the YAML configuration file.